1. Field of the Invention
The present invention relates to a stimulated Raman scattering measurement apparatus performing molecular vibration imaging by utilizing stimulated Raman scattering, the apparatus being particularly suitable for a microscope, an endoscope and the like.
2. Description of the Related Art
A stimulated Raman scattering (SRS) measurement apparatus that is one of measurement apparatuses utilizing a Raman scattering principle has been proposed in “Principle confirmation of stimulated Raman scattering microscopy” (Optics&Photonics Japan 2008, 5pC12) by Fumihiro Dake, Yasuyoki Ozeki and Kazuyoshi Ito and “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy” (SCIENCE VOL322 19 Dec. 2008 pp. 1857-1861) by Chiristian W. Freudiger, Wei Min, Brian G. Saar, Sijia Lu, Gary R. Holtom, Chengwei He, Jason C. Tsai, Jing X. Kang and X. Sunney Xie. The principle of the SRS detection apparatus is as follows.
When two light pulses whose light frequencies are mutually different are focused to a sample, a coincidence of difference between the light frequencies of the two light pulses with a molecular vibration frequency of the sample causes a phenomenon of stimulated Raman scattering at a light-focused point. The stimulated Raman scattering decreases intensity of one of the two light pulses having a higher light frequency, and increases intensity of the other one having a lower light frequency. Detection of such intensity change enables molecular vibration imaging in which vibration information of molecules of the sample is reflected. In this detection, performing intensity modulation on one of the two light pulses or providing to the one light pulse a repetition frequency that is an integral multiple of that of the other light pulse modulates the intensity change caused by the stimulated Raman scattering. Therefore, detecting the modulation of the intensity change of the other light pulse (on which the intensity modulation is not performed) by a lock-in amplifier enables detection of the stimulated Raman scattering with high sensitivity.
For such a stimulated Raman scattering measurement apparatus, it is expected that its discrimination ability for the sample may further improve, not by detecting the molecular vibration only at a specific light frequency, but by detecting a molecular vibration spectrum (hereinafter referred to as “a Raman spectrum”) in a wide vibration frequency range. Japanese patent Laid-Open No. 2010-48805 and “Label-Free and Real-Time Monitoring of Biomass Processing with Stimulated Raman Scattering Microscopy (Supporting Information)” (published by Harvard University as a research paper in 2010) disclose stimulated Raman scattering measurement apparatuses capable of scanning the light frequency of one of the above-mentioned two light pulses by using one or more optical parametric oscillators (hereinafter referred to as an “OPO”) having an oscillation wavelength variable function.
The OPO can scan a comparatively wide light frequency range by changing not only a tilt of a crystal that outputs the above one light pulse, but also temperature thereof. Moreover, “Fiber-format stimulated-Raman-scattering microscopy from a single laser oscillator” (published by Politecnico di Milano as a research paper in 2010) discloses a stimulated Raman scattering measurement apparatus capable of scanning the light frequency of one of the above two light pulses by changing temperature of a periodically poled element, such as periodically poled lithium niobate (PPLN), from which the one light pulse exits.
However, the stimulated Raman scattering measurement apparatuses disclosed in Japanese patent Laid-Open No. 2010-48805, “Label-Free and Real-Time Monitoring of Biomass Processing with Stimulated Raman Scattering Microscopy (Supporting Information)” and “Fiber-format stimulated-Raman-scattering microscopy from a single laser oscillator” gradually perform scanning of the light frequency by changing the temperature of the crystal in the OPO or of the periodically poled element, so that it takes a long time to acquire the Raman spectrum in a wide light frequency range. As methods for performing the scanning in a wide light frequency range in a short time, there are known a method of filtering wideband laser light by an acoustooptic tunable filter (AOTF) and a method of providing a chirp to two laser light pulses whose light frequencies are mutually different and controlling a time difference between the pulses to change a molecular vibration frequency. However, these methods cannot provide sufficient spectral resolution in the stimulated Raman scattering measurement apparatus.